Oxygen Abundance Distributions in Six Late-Type Galaxies Based on SALT Spectra of H II Regions

A&A 582, A35 (2015) Astronomy DOI: 10.1051/0004-6361/201526311 & c ESO 2015 Astrophysics

Oxygen abundance distributions in six late-type galaxies based on SALT spectra of H II regions

I. A. Zinchenko1,2,A.Y.Kniazev3,4,5,E.K.Grebel1, and L. S. Pilyugin1,2,6

1 Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12–14, 69120 Heidelberg, Germany e-mail: [email protected] 2 Main Astronomical Observatory, National Academy of Sciences of Ukraine, 27 Akademika Zabolotnoho St., 03680 Kyiv, Ukraine 3 South African Astronomical Observatory, PO Box 9, 7935 Observatory, Cape Town, South Africa 4 Southern African Large Telescope Foundation, PO Box 9, 7935 Observatory, Cape Town, South Africa 5 Sternberg Astronomical Institute, Lomonosov Moscow State University, Universitetskij Pr. 13, 119992 Moscow, Russia 6 Kazan Federal University, 18 Kremlyovskaya St., 420008 Kazan, Russian Federation

Received 14 April 2015 / Accepted 4 August 2015

ABSTRACT

Spectra of 34 H ii regions in the late-type galaxies NGC 1087, NGC 2967, NGC 3023, NGC 4030, NGC 4123, and NGC 4517A were observed with the South African Large Telescope (SALT). In all 34 H ii regions, oxygen abundances were determined through the “counterpart” method (C method). Additionally, in two H ii regions in which we detected auroral lines, we measured oxygen abundances with the classic Te method. We also estimated the abundances in our H ii regions using the O3N2 and N2 calibrations and compared those with the C-based abundances. With these data, we examined the radial abundance distributions in the disks of our target galaxies. We derived surface-brightness proﬁles and other characteristics of the disks (the surface brightness at the disk center and the disk scale length) in three photometric bands for each galaxy using publicly available photometric imaging data. The radial distributions of the oxygen abundances predicted by the relation between abundance and disk surface brightness in the W1band obtained for spiral galaxies in our previous study are close to the radial distributions of the oxygen abundances determined from the analysis of the emission line spectra for four galaxies where this relation is applicable. Hence, when the surface-brightness proﬁle of a late-type galaxy is known, this parametric relation can be used to estimate the likely present-day oxygen abundance in the disk of the galaxy. Key words. galaxies: abundances – HII regions – galaxies: general

1. Introduction It should be emphasized that the C method produces abundances on the same metallicity scale as the Te-method. In con- Metallicities play a key role in studies of galaxies. The present- trast, metallicities derived using one of the many calibrations day abundance distributions across a galaxy provide important based on photoionization models tend to show large discrepan- information about the evolutionary status of that galaxy and form cies (of up to ∼0.6 dex) with respect to Te-based abundances the basis for the construction of models of the chemical evolution (see the reviews by Kewley & Ellison 2008; López-Sánchez of galaxies. & Esteban 2010; López-Sánchez et al. 2012). Coupling our Oxygen abundances and their gradients in the disks of late- emission-line measurements with available line measurements type galaxies are typically based on emission-line spectra of in- from the literature or public databases, we measured the radial dividual H ii regions. When the auroral line [O iii]λ4363 is de- distributions of the oxygen abundances across the disks of six tected in the spectrum of an H ii region, the Te-based oxygen galaxies. / (O H)Te abundance can be derived using the standard equations The study of the correlations between the oxygen abundance of the Te-method. In our current study, we do not always have and other properties of spiral and irregular galaxies is impor- this information and use alternative methods where the auroral tant for understanding the formation and evolution of galax- line is not detected. In those cases, we estimate the oxygen abun- ies. The correlation between the local oxygen abundance and dances from strong emission lines using a recently suggested the stellar surface brightness (the OH – SBrelation) or surface method (called the “C method”) for abundance determinations mass density has been a subject of discussion for a long time of Pilyugin et al. (2012)andPilyugin et al. (2014a). When the (Webster & Smith 1983; Edmunds & Pagel 1984; Vila-Costas & strong lines R3, N2,andS 2 are measured in the spectrum of an Edmunds 1992; Ryder 1995; Moran et al. 2012; Rosales-Ortega ii / H region, the oxygen (O H)CSN abundance can be determined. et al. 2012; Sánchez et al. 2014). We examined the relations be- When the strong lines R2, R3,andN2 are measured in the spec- tween the oxygen abundance and the disk surface brightness in / trum, we can measure the oxygen (O H)CON abundance. the infrared W1 band of the Wide-ﬁeld Infrared Survey Explorer (WISE; Wright et al. 2010)atdiﬀerent fractions of the opti- Based on observations made with the Southern African Large cal isophotal radius R25 in our previous paper (Pilyugin et al. Telescope, programs 2012-1-RSA_OTH-001, 2012-2-RSA_OTH-003 2014b). We found evidence that the OH – SBrelation depends and 2013-1-RSA_OTH-005. on the galactocentric distance taken as a fraction of the optical Article published by EDP Sciences A35, page 1 of 14 A&A 582, A35 (2015)

Fig. 1. SDSS images of our target galaxies with marked positions of the slits used in the present investigation.

radius R25 and on other properties of a galaxy, namely its disk 2. Our galaxy sample scale length and the morphological T-type. In that study, we suggested a parametric OH – SBrelation for spiral galaxies. Our original sample of spiral galaxies for follow-up observations with the Southern African Large Telescope (SALT; Buckley In our current paper, we present results from observations et al. 2006; O’Donoghue et al. 2006) was devised based on Sloan of emission-line spectra of H ii regions in six spiral galaxies. Digital Sky Survey (SDSS) images and the fact that SALT can These observations were obtained with the South African Large observe targets with a declination δ<10 degrees and has a field Telescope as a part of our investigation of the abundance prop- of view of 8 arcmin. Each selected spiral galaxy contains a suf- erties of nearby late-type galaxies (Pilyugin et al. 2014a,b). ficiently large number of bright H ii regions distributed across the whole galaxy disk and fitting SALT’s field of view. The total We constructed radial surface-brightness profiles of our sample consists of ∼30 nearby galaxies that are located in the galaxies in the infrared W1 band using the photometric maps equatorial sky region. obtained by the WISE satellite (Wright et al. 2010). The char- Out of this sample of 58 galaxies, we have obtained spec- acteristics of the disk for each galaxy were obtained through tra of H ii regions in six galaxies (NGC 1087, NGC 2967, bulge-disk decomposition. The radial distributions of the oxygen NGC 3023, NGC 4030, NGC 4123, NGC 4517A) thus far. In abundances predicted by the parametric OH – SB relation are Fig. 1 we present the images and slit positions for those galax- compared to the radial distributions of the oxygen abundances ies. For a detailed description of the observations, we refer to determined from the analysis of the emission-line spectra of the Sect. 3. H ii regions in our target galaxies. Table 1 lists the general characteristics of each galaxy. The paper is organized as follows. Our sample of galaxies We listed the most widely used identifications for our target is presented in Sect. 2. The spectroscopic observations and data galaxies, i.e., the designations in the New General Catalogue reduction are described in Sect. 3. The photometric properties of (NGC) and in the Uppsala General Catalog of Galaxies (UGC). our galaxies are discussed in Sect. 4. The oxygen abundances are The morphological type of the galaxy and morphological type presented in Sect. 5. Section 6 contains a discussion and a brief code T were adopted from leda (Lyon-Meudon Extragalactic summary of the main results. Database; Paturel et al. 1989, 2003). The right ascension and declination were taken from the NASA/IPAC Extragalactic Throughout the paper, we use the following standard nota- Database (ned)1. The inclination of each galaxy, the position tions for the line intensities I: angle of the major axis, and the isophotal radius R25 in arcmin R = I[O III]λ4363/IHβ, = R2 I[O II]λ3727+λ3729/IHβ, 1 The NASA/IPAC Extragalactic Database (ned) is operated by the N2 = I[N II]λ6548+λ6584/IHβ, Jet Populsion Laboratory, California Institute of Technology, under con- S 2 = I[S II]λ6717+λ6731/IHβ, tract with the National Aeronautics and Space Administration. http:// R3 = I[O III]λ4959+λ5007/IHβ. ned.ipac.caltech.edu/

A35, page 2 of 14 I. A. Zinchenko et al.: Oxygen abundance distributions in six galaxies

Table 1. Adopted and derived properties of our target galaxies.

Name NGC 1087 NGC 2967 NGC 3023 NGC 4030 NGC 4123 NGC 4517A UGC 2245 UGC 5180 UGC 5269 UGC 6993 UGC 7116 UGC 7685 Morphological type, type code T SABc, 5.0 Sc, 5.2 SABc, 5.5 Sbc, 4.0 Sc, 5.0 SBdm, 8.0 Right ascension (J2000.0) [deg] 41.604852 145.513729 147.469112 180.098445 182.046296 188.117319 Declination (J2000.0) [deg] –0.498649 0.336438 0.618167 –1.100095 2.878283 0.389670 Inclination [deg], ellipticity 53.0, 0.39 21.6, 0.07 49.5, 0.35 36.9, 0.20 43.1, 0.27 50.2, 0.36 Position angle [deg] 1 151 86 38 128 23 a Isophotal radius R25 [arcmin] 1.86 1.33 1.02 1.89 1.69 1.40 Distance [Mpc] 20.1 29.5 29.4 26.4 27.3 27.8 Isophotal radius R25 [kpc] 10.86 11.41 8.72 14.51 13.42 11.32 Disk scale length in W1 band [kpc] 2.45 1.97 3.36 4.70 4.90 b −2 Logarithm of W1 brightness of disk center [L pc ] 3.056 3.315 3.234 2.557 1.955

Notes. (a) In the B band. (b) Reduced to a face-on galaxy orientation. of each galaxy were determined in our current study. The dis- setup hereafter, and one setup is from 5000–7000 Å, called the tances were taken from ned. These distances include flow cor- red setup hereafter; we combined these setups after reducing rections for the Virgo cluster, the Great Attractor, and Shapley the data. The volume phase holographic (VPH) grating GR900 Supercluster infall. The isophotal radius in kpc was estimated was used to cover the blue setup with a final reciprocal disper- −1 from the isophotal radius R25 in arcmin and the distance listed sion of ∼0.97 Å pixel and a spectral resolution resulting in a above. The characteristics of the disks (the surface brightness at FWHM of 5–6 Å (R ∼ 1000). To cover the red setup, we used the disk center in the W1 band and the disk scale length) were the VPH grating GR1300 with a final reciprocal dispersion of determined through the bulge-disk decomposition carried out in ∼0.64 Å pixel−1 and a spectral resolution with a FWHM of 3– our current paper. The surface brightness at the disk center was 4Å(R ∼ 1600). All observations were carried out between June reduced to a face-on galaxy orientation and is given in terms of −2 2012 and April 2013. L pc . During an observation at the SALT telescope the mirror re- Three galaxies of our sample are members of pairs of galax- mains at a fixed altitude and azimuth and the image of an as- ies and are included in the “Catalogue of Isolated Pairs of tronomical target produced by the telescope is followed by the Galaxies in the Northern Hemisphere” (Karachentsev 1972): “tracker”, which is located at the position of the prime focus NGC 3023 = KPG 216B, NGC 4123 = KPG 322B, NGC 4517A = (similar in operation to the Arecibo Radio Telescope). This re- KPG 344A. Two galaxies of our sample, NGC 2967 and sults in only a limited observing window per target. For this NGC 4030, are known members of galaxy groups (Fouqué et al. reason each observation consisted usually of two exposures of 1992; Garcia 1993). The galaxy NGC 4517A is a low surface about 1000 s each to fit a single SALT visibility track. For the brightness galaxy (Romanishin et al. 1983). same reason the blue and red setups were usually observed on different nights. 3. Spectroscopic observations and reduction Spectra of Ar comparison arcs and a set of quartz tungsten halogen flats were obtained immediately after each observation 3.1. Observing procedures to calibrate the wavelength scale and to correct for pixel-to-pixel The spectroscopic observations of our selected galaxies were variations. A set of spectrophotometric standard stars was ob- obtained with the multiobject spectroscopic mode (MOS) of served during twilight time for the relative flux calibration. Since the Robert Stobie Spectrograph (RSS; Burgh et al. 2003; SALT has a variable pupil size, an absolute flux calibration is Kobulnicky et al. 2003) installed at SALT. For each galaxy from not possible even using spectrophotometric standard stars. All details of the observations are summarized in Table 2.Intotal, our sample, we constructed a MOS mask, where the slit positions ii for the H ii regions were selected using gri SDSS images. A typi- 34 H regions in six galaxies were observed. cal MOS mask thus devised contained 9–12 slits for H ii regions The primary reduction of the SALT data was done with the distributed over the whole galaxy disk. The usual width of the SALT science pipeline (Crawford et al. 2010). After that, the slits was 1.5 arcsec. The general strategy of the observations was bias- and gain-corrected and mosaicked MOS data were reduced to cover the total spectral range of 3600–7000 Å in order to de- in the way described below. tect: (1) the Balmer lines Hδ,Hγ,Hβ and Hα, which were used for extinction corrections; and (2) various emission lines used for 3.2. Data reduction and line flux measurements ii iii the calculation of abundances: [O ] λ3727,3729, [O ] λ4363, 2 [O iii] λ4959,5007, [N ii] λ6548, 6584, and [S ii] λ6717,6731. Cosmic ray rejection was done using the IRAF task We chose a resolution of R = 1000–2000 to be able to re- lacose_spec (van Dokkum 2001). The wavelength cali- solve emission lines located close to each other, e.g., Hγ and bration was accomplished using the IRAF tasks identify, [O iii] λ4363, or [S ii] λ6717, and [S ii] λ6731. reidentify, fitcoord,andtransform. The spectral data In the MOS mode, the actual spectral coverage for each slit were divided by the illumination-corrected flat field to correct varies slightly depending on the X-position of a given slit rel- for pixel-to-pixel sensitivity variations of the detector. After that ative to the center of the mask. Thus, to cover the total de- 2 IRAF is distributed by the National Optical Astronomical sired spectral range of 3600–7000 Å for each slit of the mask Observatories, which are operated by the Association of Universities we had to obtain two spectral setups for each studied galaxy. for Research in Astronomy, Inc., under cooperative agreement with the One setup ranges from 3500–6500 Å, referred to as the blue National Science Foundation.

A35, page 3 of 14 A&A 582, A35 (2015)

Table 2. Journal of the observations.

Galaxy Date for Date for Exposure time Exposure time Seeing for Seeing for blue setup red setup for blue setup for red setup blue setup red setup NGC 1087 ... 2012.12.20 ... 2 × 1000 ... 1.4 NGC 2967 2013.02.02 2013.02.02 2 × 1000 2 × 1000 1.5 1.2 NGC 3023 2013.01.05 2013.01.06 2 × 1000 2 × 1000 2.5 2.3 NGC 4030 2013.03.20 ... 2 × 1000 ... 1.0 ... NGC 4123 2013.04.29 2013.03.03 2 × 1000 2 × 910 1.7 2.2 NGC 4517A 2012.06.16 ... 1 × 900 + 2 × 725 ... 1.7 ... two-dimensional spectra were extracted from the MOS images for each slit. The background subtraction was done using the IRAF task background. Since we used a multislit mask, the slits for the individual objects have a length of ∼5−20 arcsec and the background was fitted by a low-order polynomial function along the spatial slit coordinate at each wavelength. This allows us to extract the flux from the H ii regions only, with- out galactic stellar background. The spectra were corrected for sensitivity effects using the Sutherland extinction curve and a sensitivity curve obtained from observed standard star spectra. Finally, all two-dimensional spectra of a slit position obtained with the same observational setup were averaged. From each two-dimensional spectrum of the blue and red setups, one-dimensional spectra were extracted in spatial direction for each pixel along the slit. We refer to these spectra as “one- pixel-wide”. The line fluxes ([O ii]λλ3727,3729, [O iii]λ4363, Hβ,[Oiii]λ4959, [O iii]λ5007, [N ii]λ6548, Hα,[Nii]λ6584, [S ii]λ6717, and [S ii]λ6731) were then measured with IRAF or/and by fitting the lines with Gaussians following Pilyugin & Thuan (2007)andPilyugin et al. (2010a). We first consider the distribution of the emission-line fluxes along the slit. The measured fluxes in the Hβ and R3 emission lines in the blue and red one-pixel-wide spectra for slit 8 in NGC 3023 as a function of the pixel number along the slit are shown in Fig. 2. Examination of Fig. 2 shows that the position of the peak in the Hβ (and R3) emission line in the red spectrum is shifted as compared to that in the blue spectrum by approximately three pixels. This shows that the position of the slit of the Fig. 2. Fluxes in the Hβ (upper panel)andR3 (lower panel) emission red spectrum does not coincide with the position of the slit for lines in the blue (solid lines) and red (dashed lines) spectra as a function the blue spectrum. Instead they are shifted in the direction along of the pixel number along the slit for slit 8 in NGC 3023. The fluxes are in arbitrary units. the slit by approximately three pixels with respect to each other. Inspection of Fig. 2 also shows that the form of the distribution of the Hβ (and R3) emission-line flux per pixel in the red spec- ii ff the number of the presented observations of H regions varies trum di ers from that in the blue spectrum. This demonstrates from one in NGC 4030 to up to nine in NGC 1087. that the position of the slit for the red spectrum is also shifted We constructed the aperture for the blue and red spectra by in the direction perpendicular to the slit or that the position an- averaging the seven one-pixel-wide spectra near the flux max- gles of the red and blue spectrum are different. This prevents us imum. As mentioned before, the emission from the underly- from considering the lines of the blue and red spectra together. ing stellar population of the galactic disk was subtracted during Therefore we derived the abundances using the lines from the the background correction, i.e., we removed the stellar contin- blue (or red) spectrum individually. uum averaged along the spatial slit coordinates near the H ii re- The full set of lines [O ii]λλ3727, 3729, Hβ,[Oiii]λ5007, gion. Since the continuum in the spectra of our H ii regions is Hα,and[Nii]λ6548 or Hβ,[Oiii]λ5007, Hα,[Nii]λ6584, and sufficiently weak or undetectable, we neglected possible stel- [S ii]λλ6717, 6731 is needed to correct for interstellar redden- lar absorption by the stellar populations of the H ii regions. ing and to determine the oxygen abundances. For this reason, The measured emission fluxes F were corrected for interstellar only slits that provide at least one of these line sets in either reddening. We obtained the extinction coefficient C(Hβ)using the blue or red setup are chosen for further study. As the ac- the theoretical Hα-to-Hβ ratio (=2.878) and the analytical ap- tual spectral coverage for each of our slits varies slightly de- proximation to the Whitford interstellar reddening law of Izotov pending on the position of a given slit on the mask, in some et al. (1994). cases the Hβ+[O iii]λ5007 lines of the red spectra and the The dereddened emission-line fluxes in the averaged spec- Hα+[N ii]λ6548 lines of the blue spectra are shifted beyond tra of the target H ii regions are listed in Table 3 for the blue the actual spectral coverage of the slit. This is the reason why spectra and in Table 4 for the red spectra. The theoretical ratio

A35, page 4 of 14 I. A. Zinchenko et al.: Oxygen abundance distributions in six galaxies

Table 3. Dereddened emission line fluxes in units of the Hβ line flux and the extinction coefficient C(Hβ) in the blue spectra of a sample of the target H ii regions in NGC 3023.

Slit RAa Deca [O ii]λ3727,3729 [O iii]λ4363 [O iii]λ5007 [N ii]λ6584 [S ii]λ6717 [S ii]λ6731 C(Hβ) NGC 2967 17 145.503249 0.340438 2.712 ± 0.164 0.371 ± 0.033 0.772 ± 0.044 0.579 33 145.516052 0.316465 4.603 ± 0.662 1.788 ± 0.180 0.621 ± 0.041 0.704 NGC 3023 8 147.476616 0.616676 1.627 ± 0.051 0.072 ± 0.003 6.117 ± 0.200 0.297 12 147.474086 0.615905 2.600 ± 0.084 0.030 ± 0.003 3.280 ± 0.109 0.317 14 147.468182 0.619303 3.662 ± 0.120 1.654 ± 0.057 0.474 ± 0.021 0.345 NGC 4030 46 180.111544 –1.074890 2.340 ± 0.104 0.820 ± 0.034 0.849 ± 0.038 0.524 NGC 4123 14 182.032991 2.880344 2.281 ± 0.131 0.457 ± 0.031 0.858 ± 0.036 0.535 20 182.030381 2.882922 2.324 ± 0.139 0.896 ± 0.045 0.817 ± 0.039 0.403 21 182.031526 2.888546 2.322 ± 0.146 0.473 ± 0.036 0.903 ± 0.040 0.689 22 182.024433 2.888972 2.925 ± 0.166 0.781 ± 0.051 0.694 ± 0.032 0.483 ± 0.026 0.331 ± 0.017 0.485 26 182.022560 2.908112 1.771 ± 0.088 3.457 ± 0.142 0.217 ± 0.014 0.113 NGC 4517A 44 188.108016 0.381373 4.566 ± 0.224 0.936 ± 0.050 0.269 ± 0.020 0.402 56 188.116422 0.390758 2.310 ± 0.084 2.109 ± 0.096 0.305 ± 0.021 0.194

Notes. (a) In degrees (J2000).

Table 4. Dereddened emission line fluxes in units of the Hβ line flux and the extinction coefficient C(Hβ) in the red spectra of a sample of the target H ii regions in our galaxy sample.

Slit RAa Deca [O iii]λ5007 [N ii]λ6584 [S ii]λ6717 [S ii]λ6731 C(Hβ) NGC 1087 15 41.605348 –0.482317 0.794 ± 0.034 0.633 ± 0.030 0.410 ± 0.018 0.292 ± 0.013 0.353 16 41.609253 –0.490142 0.494 ± 0.020 0.750 ± 0.029 0.479 ± 0.017 0.336 ± 0.013 0.282 17 41.607925 –0.493503 0.400 ± 0.031 0.664 ± 0.026 0.377 ± 0.018 0.259 ± 0.014 0.306 23 41.604852 –0.497109 0.211 ± 0.011 0.935 ± 0.043 0.373 ± 0.017 0.278 ± 0.012 0.380 28 41.610902 –0.500035 0.429 ± 0.033 0.754 ± 0.026 0.443 ± 0.020 0.334 ± 0.015 0.252 30 41.610902 –0.486017 0.427 ± 0.017 0.696 ± 0.033 0.320 ± 0.016 0.225 ± 0.010 0.340 31 41.603092 –0.504827 0.449 ± 0.014 0.764 ± 0.027 0.275 ± 0.009 0.206 ± 0.007 0.443 32 41.604192 –0.512645 0.932 ± 0.036 0.615 ± 0.031 0.464 ± 0.027 0.322 ± 0.023 0.417 36 41.609197 –0.515824 0.406 ± 0.021 0.589 ± 0.025 0.402 ± 0.016 0.280 ± 0.016 0.251 NGC 2967 14 145.509118 0.341424 0.129 ± 0.013 0.840 ± 0.031 0.309 ± 0.013 0.226 ± 0.009 0.900 23 145.522704 0.332752 0.195 ± 0.060 0.820 ± 0.046 0.395 ± 0.024 0.270 ± 0.019 1.040 28 145.521731 0.326422 0.930 ± 0.048 0.613 ± 0.045 0.328 ± 0.031 0.216 ± 0.019 0.970 29 145.520986 0.328543 0.294 ± 0.070 0.676 ± 0.031 0.350 ± 0.029 0.252 ± 0.035 0.734 37 145.502932 0.349615 0.738 ± 0.126 0.689 ± 0.061 0.608 ± 0.049 0.295 ± 0.050 0.902 39 145.505741 0.355989 1.596 ± 0.199 0.530 ± 0.059 0.451 ± 0.064 0.229 ± 0.055 0.644 NGC 3023 8 147.476616 0.616676 5.680 ± 0.204 0.092 ± 0.004 0.122 ± 0.005 0.089 ± 0.004 0.255 12 147.474086 0.615905 3.118 ± 0.103 0.221 ± 0.008 0.225 ± 0.009 0.165 ± 0.007 0.181 14 147.468182 0.619303 1.740 ± 0.064 0.414 ± 0.017 0.456 ± 0.019 0.332 ± 0.016 0.222 18 147.480337 0.619130 3.482 ± 0.130 0.147 ± 0.006 0.225 ± 0.010 0.146 ± 0.006 0.058 19 147.463502 0.612667 2.479 ± 0.110 0.429 ± 0.041 0.500 ± 0.044 0.434 ± 0.044 0.541 21 147.457781 0.612337 1.872 ± 0.071 0.309 ± 0.016 0.484 ± 0.021 0.337 ± 0.016 0.104 22 147.486919 0.623162 2.748 ± 0.094 0.136 ± 0.009 0.240 ± 0.011 0.167 ± 0.009 0.078 NGC 4123 6 182.046371 2.878087 0.267 ± 0.012 1.523 ± 0.051 0.334 ± 0.011 0.363 ± 0.012 1.086 18 182.061021 2.864815 0.455 ± 0.043 0.816 ± 0.039 0.584 ± 0.033 0.478 ± 0.035 0.424

Notes. (a) In degrees (J2000). of [N ii]λ6584/[Nii]λ6548 is constant and close to 3 (Storey & and their flux ratio is very close to 3 (Storey & Zeippen 2000; Zeippen 2000) since those lines originate from transitions from Kniazev et al. 2004). Therefore, the [N ii]λ6548 and λ4959 lines the same energy level. Since the [N ii]λ6584 line measurements are not included in Tables 3 and 4. ii are more reliable than the [N ]λ6548 line measurements, the The uncertainty of the emission-line flux εline is estimated value of N2 is estimated as N2 = 1.33× [N ii]λ6584 unless indi- taking the uncertainty of the continuum level, errors in the line cated otherwise. Similarly, the value of R3 can be estimated as flux, and the uncertainty in the sensitivity curve into account R3 = 1.33 × [O iii]λ5007 since the [O iii]λ5007 and [O iii]λ4959 (see Kniazev et al. 2004, for details). The uncertainty of the lines also originate from transitions from the same energy level continuum, εcont, is determined in the region near the emission

A35, page 5 of 14 A&A 582, A35 (2015) line where the continuum is approximated by a linear fit. The line-flux uncertainty, εflux, is estimated as the deviation from a Gaussian profile. The uncertainty in the sensitivity curve, εsc,is less than 2−3% in all considered wavelength ranges (see, e.g., Kniazev 2012). We adopt the maximum value of the relative uncertainty as εsc = 0.03.

4. Photometry To estimate the deprojected galactocentric distance (normalized to the optical isophotal radius R25)oftheHii region from its coordinates on the celestial sphere, one needs to know the values of the inclination, i, the position angle of the major axis, PA, and the isophotal radius of a galaxy, R25. It is common practice to take those values from de Vaucouleurs et al. (1991, thereafter RC3) or from the leda database. However, some values from those sources show a significant difference for galaxies from our list. For example, the isophotal radius of NGC 4517A in the RC3 is larger by a factor of two than that given in the leda database. Therefore we obtained our own estimates of the values of i,PA, and R25 for our target galaxies. Fig. 3. Observed surface-brightness profiles of our galaxies in the g and We analyzed the publicly available photometric maps in the r bands of the SDSS photometric system and in the W1bandofthe infrared W1 band with an isophotal wavelength of 3.4 μm ob- WISE photometric system. The X-axis shows the galactocentric radius − tained by the (WISE; Wright et al. 2010) and in the g and r bands in arcsec and the Y-axis the surface brightness in mag arcsec 2.The obtained by the SDSS data release (DR) 9 (Ahn et al. 2012). We optical isophotal radius R25 is marked with an arrow. derived the surface-brightness profile and disk orientation parameters in three photometric bands for each galaxy. The determinations of the surface-brightness profile, position angle, and ellipticity were performed for each band separately in the way described in Pilyugin et al. (2014b). For NGC 3023, however, we were not able to estimate reliable values of the position angle and ellipticity for the g and r bands. Therefore, for this galaxy the values of the position angle and ellipticity obtained for the W1 band were used for the construction of the surface-brightness profiles in all three filters. It should be noted that the WISE and SDSS surveys are sufficiently deep for our surface-brightness profiles to extend beyond the optical isophotal radii R25. The obtained surface-brightness profiles are shown in Fig. 3. The adopted inclinations and position angles are given in Table 1. Fig. 4. Comparison between the measured surface-brightness profiles of The value of the isophotal radius is derived from the ob- NGC 4123 in the B band reported by Weiner et al. (2001; solid line), tained surface-brightness profiles in the g and r bands. Surface- by Micheva et al. (2013; long dashed line), and obtained here (short brightness measurements were corrected for foreground Galactic dashed line). The X-axis shows the galactocentric radius in arcmin, and −2 extinction using the AV values from the recalibration by Schlafly the Y-axis the surface brightness in mag arcsec . The arrow indicates & Finkbeiner (2011) of the extinction maps of Schlegel et al. the optical isophotal radius R25. (1998) and the extinction curve of Cardelli et al. (1989), assuming a ratio of total to selective extinction of RV = AV /EB−V = 3.1. We adopted the AV values given in the NASA Extragalactic Surface brightness profiles of the galaxy NGC 4123 in the Database ned. Afterward, we corrected the surface-brightness B band were published by Weiner et al. (2001)andMicheva et al. measurements for the inclination. The measurements in the (2013). Figure 4 shows the comparison between their profiles SDSS filters g and r were converted to B-band magnitudes, with that derived here from the photometric imaging data in the and the AB magnitudes were reduced to the Vega photometric SDSS g and r bands. system using the conversion relations and solar magnitudes of We performed a bulge-disk decomposition of the observed Blanton & Roweis (2007). First, we obtained the B-band magni- surface-brightness profiles using a purely exponential disk tudes from the g and r magnitudes (PED) approximation in the same way as in Pilyugin et al. (2014b). Exponential profiles were used to fit the observed disk = + + − − BAB g 0.2354 0.3915 [(g r) 0.6102], (1) surface-brightness profiles, and the bulge profiles were fitted with a general Sérsic profile. The observed surface-brightness B g r where the AB, ,and magnitudes in Eq. (1) are in the AB pho- profiles of five galaxies from our sample are fitted satisfactorily tometric system. Then, the AB magnitudes were reduced to the well. The upper panel of Fig. 5 shows the bulge-disk decompo- Vega photometric system sition of the galaxy NGC 1087 as an example. The measured surface profile is marked by circles. The fit to the bulge con- BVega = BAB + 0.09. (2) tribution is shown with a dotted line, the fit to the disk with a The obtained isophotal radii are given in Table 1. dashed line, and the total (bulge + disk) fitting with a solid line.

A35, page 6 of 14 I. A. Zinchenko et al.: Oxygen abundance distributions in six galaxies

Fig. 6. Diagram of [N ii]λ6584/Hα versus [O iii]λ5007/Hβ .Thesym- bols denote results for the measured H ii regions in our target galaxies. The solid line separates objects with H ii region spectra from those con- taining an AGN according to Kauffmann et al. (2003), while the dashed line represents the same separation according to the work by Kewley et al. (2001). The gray (light blue) filled circles show a large sample of emission-line SDSS galaxies from Thuan et al. (2010). Fig. 5. Patterns resulting from the bulge-disk decomposition of our target galaxies (X-axis: galactocentric radius in kpc, Y-axis: logarithm of the central surface brightness for a face-on galaxy orientation in solar circles) in the BPT classification diagram. The solid line is the 2 luminosities per pc ). Each panel shows the decomposition assuming dividing line between star-forming regions and AGNs according a purely exponential profile for the disk. The measured surface profile to Kauffmann et al. (2003), while the dashed line is the same is plotted using gray (blue) circles. The bulge contribution is shown line according to Kewley et al. (2001). Regardless of which line with a dotted line, the disk contribution with a dashed line, and the total ii (bulge + disk) fit with a solid line. is adopted, Fig. 6 shows that all our objects are H regions and their oxygen abundances can be estimated using standard techniques. / ii Table 1 lists the parameters of the disk surface-brightness pro- The Te-based oxygen (O H)Te abundances of the H regions files of those galaxies in the W1 band: the logarithm of the cen- with the detected auroral line [O iii]λ4363 were determined us- tral surface brightness of the disk in the W1 band reduced to a ing the equations for the Te-method from Pilyugin et al. (2010b, −2 face-on galaxy orientation in terms of L pc and the disk scale 2012). length in the W1 band, hW1 in kpc. Those values are parame- A new method (called the “C method”) for oxygen and ni- ters of the exponential disk approximation, described in detail in trogen abundance determinations from strong emission lines was Pilyugin et al. (2014b). recently suggested (Pilyugin et al. 2012, 2014a). In our red spec- For the galaxy NGC 3023, we could not determine a reli- tra, we measured the strong lines R3, N2,andS 2, which allowed / able disk scale length, hW1, and central surface brightness of us to determine the oxygen (O H)CNS abundances using those Σ strong lines. In some of our blue spectra, the strong lines R2, the disk, ( LW1 )0. The lower panel of Fig. 5 shows the surface- brightness-profile fit for this galaxy. The disk contribution to the R3,andN2 were measured and applied to determine the oxygen / surface brightness is close to the observed surface-brightness (O H)CON abundances. profile over a small interval of radial distances only (in fact, this is a bulge-dominated galaxy). Therefore, the values of the 5.2. The robustness and precision of the abundance disk scale length and central surface brightness of the disk are determination questionable. The emission-line fluxes measured in the one-pixel-wide spectra represent the radiation of a small part of the H ii region. One 5. Abundances would expect that the Te-based abundances in a given H ii region ff ii 5.1. Abundance determination derived from the spectra of di erent areas on the H region image should be the same or at least should be close to each other. Baldwin et al. (1981) proposed the [OIII]λ5007/Hβ vs. Is this the case for the abundances estimated from the counter- [NII]λ6584/Hα diagram (the so-called BPT classification dia- part method? To clarify this matter we have estimated the oxy- gram), which is often used to distinguish between star-forming gen abundances from the individual one-pixel-wide spectra and regions and active galactic nuclei (AGNs). The exact location considered the variations in those abundances. of the dividing line between star-forming regions and AGNs Figure 7 shows the distribution of the oxygen abundances is still controversial (see, e.g., Kewley et al. 2001; Kauffmann along the slit for the bright, extend H ii region (#8) in NGC 3023, et al. 2003). Figure 6 shows the positions of our targets (open i.e., the abundance estimated from the individual one-pixel-wide

A35, page 7 of 14 A&A 582, A35 (2015)

Our current sample of reference H ii regions (our standard reference sample from 2013) contains 250 H ii regions for which ﬀ / the absolute di erences in the oxygen abundances (O H)CON – (O/H)T and (O/H)C –(O/H)T and in the nitrogen abundances / e / NS / e / (N H)CON –(NH)Te and (N H)CNS –(NH)Te are less than 0.1 dex (Pilyugin et al. 2014a). Thus the true uncertainty in the C-based oxygen abundance may be up to around 0.1 dex even if the formal error due to the uncertainties in the strong line measurement is small. Therefore we assume that the uncertainties in the obtained oxygen abundances in our investigated H ii regions in the current paper can exceed 0.1 dex, although the formal error caused by the uncertainty in the line ﬂuxes measurement is lower.

5.3. Radial gradients The radial distribution of the oxygen abundances across the disk within the isophotal radius in each of our target galaxies was fitted with the following equation: + = + + × 12 log(O/H) 12 log(O/H)R0 CO/H (R/R25), (3) Fig. 7. Oxygen abundances along the slit for the slit 8 in the NGC 3023. + / = The circles in the upper panel show the oxygen abundances determined where 12 log(O H)R0 is the oxygen abundance at R0 0, i.e., from the individual blue one-pixel-width spectra through the Te method. the extrapolated central oxygen abundance, CO/H, is the slope of The solid line is the abundance obtained from the integrated seven- the oxygen abundance gradient expressed in terms of dex R−1, pixel-width spectrum through the T method. The circles in the lower 25 e and R/R25 is the fractional radius, i.e., the galactocentric distance panel show the oxygen abundances determined from the individual red normalized to the disk’s isophotal radius R . C 25 one-pixel-width spectra through the NS method. The solid line is the NGC 1087. The strong lines Hβ,[Oiii]λλ4959, 5007, Hα, abundance obtained from the integrated seven-pixel-width spectrum ii ii through the C method. [N ]λλ6548, 6584, and [S ]λλ6717, 6731 were measured in NS nine red spectra. In those H ii regions, we derived the oxygen abundance (O/H) . The resulting oxygen abundances are listed spectra as a function of the number of the pixel along the slit. CNS in Table 5. Those abundances are shown by the filled circles in The auroral line R = [O iii]λ4363 is detected in around 20 indi- panel a of Fig. 8. vidual one-pixel-wide spectra of this H ii region. The circles in There are three SDSS spectra of H ii regions in the galaxy the upper panel of the Fig. 7 show the oxygen abundances deter- NGC 1087 in data release 7 (DR7, Abazajian et al. 2009), mined from the individual blue one-pixel-wide spectra through namely Sp 409-51871-237, Sp 1069-52590-193, and Sp 1511- the T method. The solid line is the T -based abundance obtained e e 52946-192. (The SDSS spectrum number consists of the SDSS from the seven-pixel-wide spectrum. The circles in the lower plate number, the modified Julian date of the observation, and panel of the Fig. 7 show the oxygen abundances determined the number of the fiber on the plate.) Since SDSS data release 10 from the individual red one-pixel-wide spectra through the (DR10, Ahn et al. 2014) reported line measurements in only one C method. The solid line represents the abundance obtained NS spectrum, we used the SDSS spectra from DR7. The oxygen from the seven-pixel-width spectrum through the C method. NS (O/H) abundances inferred using the SDSS spectra are shown Inspection of the upper panel of Fig. 7 confirms that the CNS with open (blue) circles in panel a) of Fig. 8. T -based abundances are independent from the position in e The best linear fit to all the data points (12 points) with galac- the H ii region image at which the measurement was taken. tocentric distances smaller than the isophotal R radius is Examination of the lower panel of Fig. 7 shows that the abun- 25 dances determined from the individual one-pixel-wide spectra 12 + log(O/H) = 8.61 ± 0.02 − 0.285 ± 0.056 × (R/R25), (4) through the C method are close to each other and are close to the abundance obtained from the integrated seven-pixel-wide spec- with a mean deviation of 0.034 dex around the relationship. The trum. A similar picture was found for other H ii regions. Thus, obtained relation is shown by a solid line in panel a) of Fig. 8. the C-based abundances are robust and independent from the NGC 2967. The strong lines Hβ,[Oiii]λλ4959,5007, Hα, specific area covered by the measurement in an H ii region im- [N ii]λλ6548,6584 and [S ii]λλ6717,6731 were measured in six / age, i.e., the C method produces a reliable oxygen abundance red spectra. Oxygen (O H)CNS abundances were then inferred for even if a spectrum of only part of an H ii region is used. those H ii regions. These abundances are shown with filled cir- The scatter in individual C-based abundances is even lower cles in panel b) of Fig. 8. The strong lines [O ii]λλ3727,3729, than the scatter in individual Te-based abundances, i.e., the for- Hβ,[Oiii]λλ4959,5007, Hα,and[Nii]λ6584 were measured mal uncertainty in the C-based abundances is lower than that in two blue spectra of H ii regions in the galaxy NGC 2967 / in the Te-based abundances. This can be attributed to the fact and were used to estimate oxygen (O H)CON abundances. Those that the measurements of the weak auroral lines used in the abundances are shown with black filled squares in panel b) of Te method can involve larger errors than the measurements of Fig. 8. The obtained oxygen abundances are given in Table 5. the strong lines used in the C method. It should be noted, how- There are three SDSS spectra (Sp 476-52314-622, Sp 266- ever, that the true uncertainty in the C-based oxygen abundance 51630-387, and 266-51602-394) of H ii regions in the galaxy / depends on the accuracy of the strong-line measurements in the NGC 2967 in DR7. The oxygen (O H)CNS abundances derived spectrum of the target H ii region as well as on the reliability using the SDSS spectra are shown with gray (blue) open circles of the abundance determinations in the reference H ii regions. in panel b) of Fig. 8.

A35, page 8 of 14 I. A. Zinchenko et al.: Oxygen abundance distributions in six galaxies

ii / Table 5. Oxygen abundances in the H regions in the disks of our sam- We derived the oxygen (O H)CON abundance, finding the to- ple of galaxies. tal nitrogen flux N2 to be 4 [N ii]λ6548. This abundance is shown with the black filled square in panel c of Fig. 8.The a iii ii Slit R/R25 12+log(O/H) Method Spectrum strong lines Hβ,[O ]λλ4959, 5007, Hα,[N ]λλ6548, 6584, NGC 1087 and [S ii]λλ6717, 6731 were measured in seven red spectra and / ii 15 0.527 8.44 CNS red used to infer the oxygen (O H)CNS abundance for those H re- 16 0.359 8.48 CNS red gions. The total nitrogen fluxes were determined to be N2 = 17 0.232 8.50 CNS red 1.33[N iiλ6584. Those abundances are shown with black filled 23 0.050 8.61 CNS red circles in panel c) of Fig. 8. 28 0.327 8.50 CNS red There are four SDSS spectra (Sp 480-51989-056, Sp 481- 30 0.506 8.52 CNS red 51908-289, Sp 267-51608-384, and Sp 267-51608-389) of 31 0.231 8.55 CNS red H ii regions in the galaxy NGC 3023. Since there is a large dis- 32 0.444 8.42 C red NS crepancy between the line fluxes reported in DR7 and DR10, 36 0.603 8.48 CNS red NGC 2967 these SDSS spectra were not used. The best linear fit to the data points (nine points) with galac- 14 0.232 8.66 CNS red 17 0.554 8.53 CON blue tocentric distances smaller than the isophotal R25 radius is 23 0.454 8.60 CNS red 12 + log(O/H) = 8.32 ± 0.04 − 0.315 ± 0.078 × (R/R25), (6) 28 0.575 8.45 CNS red 29 0.494 8.55 CNS red with a mean deviation of 0.047 dex around the relationship. The 33 0.936 8.35 CON blue obtained relation is plotted with a solid line in panel c) of Fig. 8. 37 0.785 8.44 CNS red NGC 4030. Unfortunately, the spectral setup used for 39 0.949 8.36 CNS red this galaxy only covers the wavelengths of the Hα and NGC 3023 [N ii]λ6584 lines for one of the slits. Therefore, the strong 08 0.449 8.16 Te blue lines [O ii]λλ3727,3729, Hβ,[Oiii]λλ4959,5007, Hα,and 8.10 CNS red [N ii]λ6584 were measured in only one blue spectrum of an 12 0.336 8.16 T blue e H ii region in the galaxy NGC 4030. The oxygen (O/H) abun- 8.22 C red CON NS dance was estimated using the measured strong lines. The in- 14 0.110 8.34 CON blue 8.30 CNS red ferred abundance is shown with black filled squares in panel d of 18 0.665 8.09 CNS red Fig. 8. There are six SDSS spectra (Sp 285-51930-042, Sp 285- 19 0.575 8.22 CNS red 51663-044, Sp 285-51930-049, Sp 285-51663-058, Sp 331- 21 0.826 8.09 CNS red 52368-405, and Sp 2892-54552-293) of H ii regions in the 22 1.117 7.99 CNS red / galaxy NGC 4030 in DR7. The oxygen (O H)CNS abundances NGC 4030 based on the SDSS spectra are shown with gray (blue) open cir- 46 0.910 8.52 CON blue cles in panel d of Fig. 8. NGC 4123 The best linear fit to all the data points (seven points) with 6 0.001 8.60 CNS red galactocentric distances smaller the isophotal R25 radius is 14 0.528 8.54 CON blue 18 0.711 8.47 CNS red 12 + log(O/H) = 8.78 ± 0.01 − 0.286 ± 0.017 × (R/R25), (7) 20 0.629 8.51 CON blue 21 0.646 8.54 C blue with a mean deviation of 0.009 dex from the relationship. The ON relation is shown with a solid line in panel d) of Fig. 8. 22 0.889 8.44 CNS blue NGC 4123. The strong lines [O ii]λλ3727,3729, Hβ, 8.47 CON blue iii ii 26 1.399 8.26 CON blue [O ]λλ4959,5007, Hα,and[N ]λ6584 were measured in five NGC 4517A blue spectra of H ii regions in the galaxy NGC 4123. The / 44 0.603 7.95 CON blue oxygen (O H)CON abundances based on those spectral data are 56 0.082 8.32 CON blue shown with black filled squares in panel e) of Fig. 8. The sulfur lines [S ii]λλ6717,6731 were also measured in the blue spec- Notes. (a) Standard error for the C-based methods is around 0.1 dex. See trum of one H ii region in the galaxy NGC 4123. This allowed Sect. 5.1 for more details. / us to obtain the oxygen (O H)CNS abundance for this particu- lar H ii region. The strong lines Hβ,[Oiii]λλ4959, 5007, Hα, [N ii]λλ6548, 6584, and [S ii]λλ6717, 6731 were measured in The best linear fit to all the data points (11 points) with galac- two red spectra. The inferred oxygen (O/H) abundances are tocentric distances smaller the isophotal R radius is CNS 25 shown with the black filled circles in panel e) of Fig. 8. It should be noted that the H ii region at the center of NGC 4123 (Slit 06) 12 + log(O/H) = 8.75 ± 0.02 − 0.414 ± 0.030 × (R/R ), (5) 25 is located close to the line dividing AGNs and star-forming re- with a mean deviation of 0.026 dex around the relationship. The gions in the BPT classification diagram. resulting relation is represented with a solid line in panel b) of Spectra of the region near the center of the NGC 4123 Fig. 8. were observed by Kehrig et al. (2004) and by SDSS (Sp 517- 52024-504). We obtained abundances of 12+log(O/H) = 8.56 NGC 3023. The auroral line R = [O iii]λ4363 was de- CNS and 12+log(O/H) = 8.63 using the spectral measurements of tected in two blue spectra of H ii regions in the disk of CON NGC 3023. The oxygen abundances in those H ii regions Kehrig et al. (2004). Moreover, we measured an abundance of 12+log(O/H)C = 8.63 using the DR10 line fluxes. were determined through the direct Te method. Those abun- NS dances are shown with plus signs in panel c) of Fig. 8. The best linear fit to all the data points (ten points) with The strong lines [O ii]λλ3727, 3729, Hβ,[Oiii]λλ4959, 5007, galactocentric distances smaller than the isophotal R25 radius is Hα,and[Nii]λ6548 were measured in one blue spectrum. 12 + log(O/H) = 8.61 ± 0.01 − 0.164 ± 0.025 × (R/R25), (8)

A35, page 9 of 14 A&A 582, A35 (2015)

Fig. 8. Radial distributions of oxygen abundances in the disks of our target galaxies. The plus signs are abundances derived through the Te method, the circles are abundances obtained through the CNS method, and the squares are those inferred through the CON method. The filled symbols show abundances based on our SALT spectra, the open (blue) symbols are abundances based on spectra from the literature (see text). The solid line in each panel is the best linear fit to the data points with galactocentric distances less than the isophotal R25 radius. (A color version of this figure is available in the online version.) with a mean deviation of 0.025 dex. This relation is represented We corrected these galactocentric distances for the galaxy dis- with a solid line in panel e) of Fig. 8. tance adopted here and used the resulting values. Furthermore, ii there are two SDSS spectra (Sp 289-51990-627 and Sp 290- NGC 4517A. The strong lines [O ]λλ3727,3729, Hβ, ii [O iii]λλ4959, 5007, Hα,and[Nii]λ6584 were measured in the 51941-350) of H regions in NGC 4517A. The abundances blue spectrum of an H ii region in the galaxy NGC 4517A. In based on the SDSS and Romanishin et al.’s data are shown with another spectrum, the line [N ii]λ6584 is out of our spectral gray (blue) open circles in panel f) of Fig. 8. range, but the line [N ii]λ6548 is included. We derived the oxy- The best linear fit to all the data points (eight points) with / galactocentric distances smaller than the isophotal R25 radius is gen (O H)CON abundance in these spectra. The total nitrogen flux N was determined to be N = 1.33[N ii]λ6584 in the former case 2 2 12 + log(O/H) = 8.35 ± 0.01 − 0.663 ± 0.033 × (R/R25), (9) andtobeN2 = 4[Nii]λ6548 in the latter case. These abundances are shown with black filled squares in panel f) of Fig. 8. with a mean deviation of 0.023 dex. This relation is indicated by a solid line in panel f) of Fig. 8. Romanishin et al. (1983) reported emission-line ratios [S ii](λ6717 + λ6731)/Hα,Hα/[N ii](λ6548 + λ6584), and [O iii](λ4959+λ5007)/Hβ obtained from photographic spectra 5.4. Comparison between the distributions of the abundances determined through the C method of four H ii regions in NGC 4517A. We estimated the oxygen / / and via the O3N2 and N2 calibrations (O H)CSN andnitrogen(NH)CSN abundances from those strong lines. Romanishin et al. (1983) did not provide the positions of Many calibrations based on photoionization models and/or the observed H ii regions, but listed the deprojected radii instead. H ii regions with abundances determined through the direct

A35, page 10 of 14 I. A. Zinchenko et al.: Oxygen abundance distributions in six galaxies

Fig. 9. Comparison of the radial distributions of oxygen abundances in the disks of our target galaxies determined through the C method (filled dark [black] circles), via the O3N2 calibration (open gray [red] circles), and through the N2 calibration (plus signs). The solid line in each panel is the best linear fit to the (O/H)C abundances (the same as in Fig. 8). (A color version of this figure is available in the online version.)

Te-method were suggested for nebular abundance determina- and photoionization models. Pure empirical O3N2 and N2 cal- tions (Pagel et al. 1979; Alloin et al. 1979; Dopita & Evans ibrations, i.e., based on H ii regions with abundances deter- 1986; McGaugh 1991; Zaritsky et al. 1994; Pilyugin 2000, mined through the direct Te-method, were recently presented by 2001; Kewley & Dopita 2002; Pettini & Pagel 2004; Pilyugin Marino et al. (2013). Thus we determined the (O/H)O3N2 abun- & Thuan 2005; Tremonti et al. 2004; Stasinska´ 2006, among dances in our H ii regions also using the O3N2 calibration of many others). The discrepancies between metallicities of a Marino et al. (2013), i.e., given H ii region derived using different calibrations can be large, up to ∼0.6 dex (see reviews by Kewley & Ellison 2008; 12 + log(O/H)O3N2 = 8.533 − 0.214 × O3N2, (10) López-Sánchez & Esteban 2010; López-Sánchez et al. 2012). However, one would expect that all the calibrations based on the where O3N2 = log[([O III]λ5007/Hβ)/[N II]λ6584/Hα)], and / abundances in H ii regions determined through the Te method the (O H)N2 abundances using their N2 calibration should produce the abundances close to each other. Since the C method produces abundances on the same metallicity scale as 12 + log(O/H)N2 = 8.743 + 0.462 × N2, (11) the Te-method, those abundances should be close to the abundances produced by other calibrations based on the direct abun- where N2 = log([N II]λ6584/Hα). dances in H ii regions. Figure 9 shows the comparison of the radial distributions of the oxygen abundances in the disks of our target galaxies deter- The O3N2 and N2 calibrations suggested by Pettini & Pagel mined through the C method (filled dark [black] circles), via the (2004) are widely used. The original O3N2 and N2 calibra- O3N2 calibration (open gray [red] circles), and through the N2 tions are hybrid calibrations, i.e., they are based on both H ii recalibration (plus signs). The solid line in each panel is the best gions with abundances determined through the direct Te-method linear fit to the (O/H)C abundances (the same as in Fig. 8).

A35, page 11 of 14 A&A 582, A35 (2015)

are within ∼0.1 dex. This supports our estimation of the uncertainties in the abundances discussed in Sect. 5.2.

6. Discussion The radial distributions of the oxygen abundances across the disks of all the galaxies of our sample are well ﬁtted by linear relationships within the isophotal radius (with the abundances on the logarithmic scale). The mean deviation from the relationship is less than 0.05 dex for each galaxy. The values of the radial abundance gradient vary by a factor of ∼4 among the galaxies of − −1 − our sample; from 0.164 dex R25 for NGC 4123 to 0.663 dex −1 R25 for NGC 4517A. The correlation between the local oxygen abundance and the stellar surface brightness (OH – SB relation) or surface mass density has been discussed in many studies (Webster & Smith 1983; Edmunds & Pagel 1984; Vila-Costas & Edmunds 1992; Ryder 1995; Moran et al. 2012; Rosales-Ortega et al. 2012; Sánchez et al. 2014). In our previous paper (Pilyugin et al. 2014b), we examined the relations between the oxygen abundance and the disk surface brightness in the infrared W1bandof WISE at diﬀerent fractions of the optical isophotal radius R25. We found evidence that the OH – SBrelation varies with galactocentric distance and depends on the disk scale length and the morphological T-type of a galaxy. We derived a general parametric relation between abundance and surface brightness in the W1 band, O/H = f (SB) for spiral galaxies of type Sa– Sd,

12 + log (O/H) = 7.732 + 0.303 x + 0.290 x2 + + 0.288 0.120 x 2 −0.139 x )log(ΣL W1 x (12) + 0.0418−0.0323 x − 0.0022 x2 h ii W1 Fig. 10. Comparison of the oxygen abundances in the individual H re- − + + 2 gions of our sample determined through the C method with oxygen 0.0122 0.0404 x 0.0088 x T, abundances obtained through the O3N2 calibration (upper panel)and = through the N2 calibration (lower panel). The solid line indicates a one- where x r/R25 is the fractional radius expressed in terms of to-one correspondence. The dashed lines are shifted by ±0.1dex. the isophotal radius of a galaxy (R25), (ΣL)x is the disk surface brightness, hW1 the radial disk scale length, and T the morphological T-type. It is interesting to compare the radial distribu- Figure 10 shows the comparison of the oxygen abundances tions of oxygen abundances predicted by this relationship to the in the individual H ii regions of our sample determined through radial abundance trends traced by the oxygen abundances in the the C method with the oxygen abundances obtained through the H ii regions in the disks of our sample of galaxies. O3N2 calibration (upper panel) and through the N2 calibration Figure 11 shows the comparison between the radial distribu- (lower panel). tions of the oxygen abundances predicted by the O/H = f (SB) Figures 9 and 10 demonstrate that the (O/H)O3N2 abundances relation, Eq. (12), and the abundances obtained from the anal- are in satisfactory agreement (within 0.1 dex) with the (O/H)C ysis of the emission-line spectra of H ii regions for four of our abundances for H ii regions with metallicities 12+log(O/H) ∼> galaxies, NGC 1087, NGC 2967, NGC 4030, and NGC 4123. 8.1. However, a small systematic difference, around 0.05 dex, The O/H = f (SB) relation cannot be applied to the other two between (O/H)O3N2 and (O/H)C abundances seems to exist; in galaxies of our sample. It was noted above that we could not de- the sense that the (O/H)O3N2 abundances are slightly lower termine a reliable disk scale length hW1 and surface brightness than the (O/H)C abundances. A large disagreement between at the center of the disk of the galaxy NGC 3023 since the disk (O/H)O3N2 and (O/H)C abundances for H ii regions with metal- contribution to the surface brightness is close to the observed licities 12+log(O/H)C ∼< 8.1 is not surprising since the O3N2 surface-brightness profile over a small range of radial distances calibration of Marino et al. (2013) is constructed for H ii regions only (see lower panel of Fig. 5). NGC 4517A is a Sdm galaxy with metallicities 12+log(O/H) ∼> 8.1 and does not work at low (with morphological type T = 8), whereas the O/H = f (SB)re- metallicities. The differences between the (O/H)N2 and (O/H)C lation was derived for spiral galaxies of the types Sa– Sd (i.e., abundances exceed 0.1 dex for some H ii regions with metallici- for a range of morphological types from T ∼ 1toT ∼ 7). ties 12+log(O/H) ∼> 8.1. This may suggest that the O3N2 calibra- Inspection of Fig. 11 shows that the oxygen abundances pre- tion of Marino et al. (2013) provides more reliable abundances dicted by the parametric O/H = f (SB) relation are rather close than their N2 calibration. to the abundances obtained from the analysis of the emission- In summary, the comparison between C-, O3N2-, and N2- line spectra of H ii regions of the galaxies of the present sample based abundances in our target H ii regions allows us to sug- where the OH – SBrelation is applicable. The discrepancy usu- gest that the uncertainties in the obtained (O/H)C abundances ally does not exceed 0.1 dex. Thus, the parametric O/H = f (SB)

A35, page 12 of 14 I. A. Zinchenko et al.: Oxygen abundance distributions in six galaxies

Fig. 11. Radial distributions of oxygen abundances in the disks of the spiral galaxies of our sample. The circles represent the abundances of the individual H ii regions (the same as in Fig. 8). The line in each panel shows the abundance distribution predicted by the relation between abundance and surface brightness in the W1 band, Eq. (12).

relation can be used for a rough estimation of the oxygen abun- of Marino et al. (2013). The (O/H)O3N2 abundances are in sat- dances in the disks of spiral galaxies. isfactory agreement (within 0.1 dex) with the (O/H)C abundances for H ii regions with metallicities 12+log(O/H) ∼> 8.1. However, a small systematic difference, around 0.05 dex, be- Summary tween (O/H)O3N2 and (O/H)C abundances seems to exist in the / Spectra of H ii regions in six late-type galaxies were observed sense that the (O H)O3N2 abundances are slightly lower than the / ff / with the South African Large Telescope (SALT). The auroral (O H)C abundances. The di erences between the (O H)N2 and (O/H) abundances are larger than 0.1 dex for some H ii re- line [O iii]λ4363 was detected in two spectra. The Te-based C gions. This may suggest that the O3N2 calibration of Marino oxygen (O/H)T abundances in these two H ii regions were de- e et al. (2013) provides more reliable abundances than their rived using the equations of the standard Te-method. The oxygen abundances of the other H ii regions were estimated from strong N2 calibration. emission lines through the recently suggested “counterpart” We determined the abundance gradients in the disks of our method (C method). When the strong lines R3, N2,andS 2 were six late-type target galaxies. The radial distributions of the oxy- / gen abundances across the disks of all the galaxies of our sample measured in our spectra, oxygen (O H)CNS abundances could be obtained. When, on the other hand, the strong lines R2, R3,and are well fitted by linear relationships within the isophotal radius / (with abundances on a logarithmic scale). The mean deviation N2 were available, then oxygen (O H)CON abundances were determined. Moreover, we also inferred oxygen abundances of the from the relationship is less than 0.05 dex for each galaxy. The H ii regions in our target galaxies with available spectral mea- values of the radial abundance gradient vary by a factor of ∼4 − −1 surements from the literature or from the SDSS spectroscopic among the galaxies of our sample, i.e., from 0.164 dex R25 for − −1 data base through the C method. NGC 4123 to 0.663 dex R25 for NGC 4517A. We derived oxygen abundances from the individual one- We derived surface-brightness profiles in three photometric pixel-wide spectra and considered the variations in those abun- bands (the W1bandofWISEandtheg and r bands of the SDSS) dances. The abundances determined with the C method from for each galaxy using publicly available photometric imaging the individual one-pixel-wide spectra are close to each other and data. The characteristics of the disks (the surface brightness at are close to the abundances obtained from the integrated seven- the disk center and the disk scale length) were found through pixel-wide spectrum. This can be considered as supporting ev- bulge-disk decomposition. Using the photometric parameters of idence for the robustness and precision of the C-based abun- the disks, the oxygen abundance distributions were estimated dances, which are independent of the area in the H ii region im- from the relation between abundance and surface brightness of age that is measured. In other words, the C method produces a the disk in the W1 band, O/H = SB, which had been obtained reliable oxygen abundance even if a spectrum of only a part of for spiral galaxies in our previous study. The oxygen abundances an H ii region is used. predicted by the O/H = SBrelation are rather close to the bun- We also determined the (O/H)O3N2 and (O/H)N2 abundances dances determined from the analysis of the emission-line spectra in our target H ii regions using the O3N2 and N2 calibrations of the H ii regions in the galaxies of the present asample where

A35, page 13 of 14 A&A 582, A35 (2015) the OH – SBrelation is applicable. The discrepancy is usually Izotov, Y. I., Thuan, T. X., & Lipovetsky, V. A. 1994, ApJ, 435, 647 not larger than 0.1 dex. Thus, the parametric O/H = f (SB) rela- Karachentsev, I. D. 1972, Soobshcheniya Spetsial’noj Astrofizicheskoj tion can be used for a rough estimation of the oxygen abundances Observatorii, 7, 1 Kauffmann, G., Heckman, T. M., Tremonti, C., et al. 2003, MNRAS, 346, 1055 in the disks of spiral galaxies. Kehrig, C., Telles, E., & Cuisinier, F. 2004, AJ, 128, 1141 Kewley, L. J., & Dopita, M. A. 2002, ApJS, 142, 35 Acknowledgements. The observations reported in this paper were obtained with Kewley, L. J., & Ellison, S. L. 2008, ApJ, 681, 1183 the Southern African Large Telescope (SALT). L.S.P., I.A.Z., and E.K.G. ac- Kewley, L. J., Dopita, M. A., Sutherland, R. S., Heisler, C. 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